Structure and Function of the Muscular System

The human body has more than 600 muscles. The three types of muscle tissue are skeletal, visceral, and cardiac (Table 15-1). Muscles do the following:

 Aid in movement

 Provide and maintain posture

 Protect internal organs

 Provide movement of blood, food, and waste products through the body

 Open and close body openings

 Produce heat (Fig. 15-1)

Muscle contraction is the movement of muscles when stimulated. Tonus is the muscle’s ability to maintain slight, continuous contraction. Muscles can be stimulated electrically, mechanically, or chemically. Muscles are flaccid, or soft, when not contracted. Muscle tissue has several unique characteristics:

Skeletal muscles make up more than 40% of a person’s body weight. They increase in size and weight with exercise and decrease with inactivity. Muscle size and strength vary among people because of genetic differences and nutritional and exercise habits. Muscles are attached to bones by tendons, which are narrow strips of dense connective tissue. Muscles are named according to their location, related bones, shape, action, or size (Fig. 15-2).

Skeletal muscle tissue looks striated, or banded, under the microscope (Fig. 15-3). Striated muscle is made of bundles of fine fibers. The number of muscle fibers does not increase much after birth. Increase in muscle mass is due to an increase in the size of the fibers. Fascia is a layer of fibrous connective tissue that separates individual muscles.

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